More SSR confusion
I would like to add to Leggs11's comments.
According to the UK AIP (page ENR 1-5-1, I think) ATC may authorise a/c to transit controlled airspace IFR without mode C, on an occasional basis, and subject to traffic. However this authorisation shall not be given to aircraft that regularly require access to the particular airspace.
This is from memory, certainly not verbatim, but you get the gist!
Interpret this as you will.